Flaws required to be detected during PAUT inspection of tubes are often in the form of elongated defects having an angle θ with respect to the tube axis. Response from such defects is usually calibrated using a calibration tube which has a series of well-defined notches scribed on the outside diameter (OD) and inside diameter (ID) of the tube. The echo response amplitude from such notches is strongly peaked in the direction perpendicular to the length of the notch.
In existing practice, the echo response amplitude is calibrated for each notch on the calibration tube by receiving the scattered ultrasound energy in a direction perpendicular to the length of the notch. In this way, satisfactory calibration is achieved for actual angles of the calibration notches, but a calibration notch is required for every notch angle. Reliable calibration cannot be obtained for intermediate notch angles because of the non-linearity of the system. Thus the calibrations in existing practice exhibit “gaps” in the reliability of calibration with respect to notch angle.
In existing practice, the effect of the angular calibration gaps has been minimized by having a large number of calibration notches at closely spaced angular increments. However, such a method suffers disadvantages in the expense of machining the large number of notches required, and in the loss of productivity due to the time taken to perform calibrations on many notches.
Another disadvantage of calibrations in existing practice is that they rely on a single notch for calibration. It is known that multiple notches machined according to the same specification exhibit some variation with respect to detection amplitude. Accordingly, existing methods are very susceptible to these variations since the calibration depends entirely on a single notch.
Therefore there exists a need for a “gapless” calibration method which uses a small number of calibration notches with a reliable method of interpolating between calibration notch angles, and in which amplitude variations from a single notch are appropriately averaged. In view of their reliance on intermediate notches and on unreliable measurements from a single notch, existing methods are not able to provide gapless calibration.